1. Field of the Invention
The present invention relates to a method of locating a mobile telephone unit within a cellular service area, and more particularly to a method of predicting the location of a CDMA mobile unit based upon the probability of its being at a particular location of the service area using an algorithm providing a likelihood estimation of the mobile unit's location in response to a sequential set of attributes observed by the mobile unit and reported back to a base station.
2. Description of Related Art
A cellular telephone system must be able to locate a mobile unit within a cellular service area under various RF propagation conditions such, for example, when an E911 call is made from the mobile unit. Conventional methods for locating a mobile unit are typically based on either a triangulation technique which requires signals from three or more base stations within a designated service area, or an angle of arrival technique which requires at least two base stations. In many areas, the number of base stations the mobile unit can detect is less than two. Furthermore, both the triangulation and angle of arrival techniques inherently suffer from inaccuracies and signal fading which result from multi-path propagation.
In the above-noted related patent application U.S. Pat. No. 6,496,701 entitled “Pattern Recognition-Based Geolocation”, RF characteristics pertaining to one or more pilot signals radiated from a base station and specific to a particular location within the service area are detected by a mobile unit and transmitted back to a base station where they are matched to a known set of RF characteristics and other information obtained from making attribute information measurements at all the grid points (sub-cells) in a cellular service area and which are then stored in a database located, for example, in a base station server.
In the above-noted related patent application U.S. Ser. No. 09/294,997 entitled “A Bayesian-Update Based Location Prediction Method For CDMA systems”, the invention is directed to a method of estimating, by a Bayesian probability algorithm, the location of a mobile unit in the service area of a CDMA cellular telephone system using a model based approach which, among other things, simplifies the generation of a database containing a pilot signal visibility probabilities. This eliminates the need for repeated attribute measurements at all of the grid points in the service area.
In the above-noted related patent U.S. Pat. No. 6,263,608 entitled “Geolocation Estimation Method For CDMA Terminals Based On Pilot Strength Measurements”, the invention is directed to a method of estimating the location of a mobile unit in the service area of a CDMA cellular telephone system also using a model based approach, but which now eliminates the need for a stored database containing pilot signal visibility probabilities for all of the grid points or sub-cells in the cellular service area. The estimation procedure is based entirely on analytical results involving one or more key approximations derived, for example, from an integrated model of the wireless communications system, its RF environment, and attribute measurement.